Black & Tan 332iT

[heinzboehmer]

Awesome! It’s been a long time since I cheated on my homework in dynamics, so at some point I’ll need to know the critical changes to apply this to my sedan 😂. Who can keep up with that syllabus anyway? Hopefully I’ll just plug and chug with a new weight bias.

The jury is still out until I get my alignment, but I might ping you about those rear LCAs!

I regret not doing camber arms last time I had my subframe out. Might be worth to just do them now before you assemble the sedan.

[tlow98]

Awesome! It’s been a long time since I cheated on my homework in dynamics, so at some point I’ll need to know the critical changes to apply this to my sedan 😂. Who can keep up with that syllabus anyway? Hopefully I’ll just plug and chug with a new weight bias.

The jury is still out until I get my alignment, but I might ping you about those rear LCAs!

[Bry5on]

So I finally finished my project of modeling the roll centers at each end of the car, along with the roll axis and CG so I can calculate total roll and correctly set my front/rear roll couples for steady state handling. For the setup I've modeled, I'm on stock e46 M3 springs with flipped front camber plates and maxed out rear lower control arm negative camber. I've also got Turner RACP plates which drop the subframe by I think 1/8".

As you'll see below in the following images, the e46 has a pretty steep roll axis curve. What this gives the car, is natural stabilizing tendencies on dynamic corner and throttle changes, and a tendency to provide front end grip on steering input changes (contrast to a 911 which is a bit different/backwards). The way to think about roll couple geometries is all about dynamic transitions. When your car is going through an instantaneous change to steering/cornering load, your suspension takes time to settle into its steady state. At initial steering input, your roll stiffness comes from your suspension geometry, rather than your springs and sway bars. As you settle into the corner, your damper settings control the transition of load from your initial suspension geometry loads, to your steady state spring/roll bar driven loads.

I'm over-generalizing a bit, but the skinny is that the roll axis is very important for dynamic transitions, and is also extremely important for calculating ideal roll couples (FRC or Front Roll Couple) in that wonderful FCM Ride Harmony spreadsheet.

I measured my car, an M3 converted wagon with 48.5/51.5 F/R weight bias, to have a CG at 540mm, a front roll center height of 39mm, and a rear roll center height of 284mm. This means that the roll moment arm for the front suspension is 501mm and the rear suspension is 256mm. In other words, the front roll moment is 1.96x the rear one. Using these numbers, we can now calculate the ideal steady-state roll couple for my car on square tires, with the untrue-but-generally-practical assumption that the roll center does not move from here. What's the answer? A Front Roll Couple of ~66% is the theoretically balanced steady-state chassis that will yield maximum corner holding. This is impractical for most folks, as a perfectly balanced car can feel skittish when on the power, and will not tend to understeer at the limit for safety, among other reasons.

Now that the weather has gotten a bit more dry, I've noticed that my car does indeed have a tendency to understeer at the limit with my current FRC of 71.5% - I'll be installing the CSL rear bar I have to bring this down to an FRC of 69.9% to see how that does. Wish me luck!

edit: Also note, that the more you lower the front of the car relative to the rear, the higher your FRC needs to be to compensate!

Full Model isometric view


Front Model



Rear Model



Side/Axis Model



Roll couple numbers

[Bry5on]

Today was a long day under the car, not a lot of pictures but a fellow local e46 friend @tflow (tagging never seems to work with you man) came by to lend me a hand with this one. Thank you! The list of things done today:

• Installed rebuilt 3.62:1 wavetrac diff, REM polished gears from diffsonline
• Replaced my brand damn new rear lower control arms with ECS adjustable ones
• Replaced rear sway bushings and links
• Replaced the lower shift pivot. I hate that thing.
• Replaced inner half shaft CV joints
• Replaced driveshaft rear CV joint
• Dropped the subframe and drilled and tapped the holes for the upcoming vincebar

I once again chickened out on doing solid subframe bushings. I decided at the last minute to keep the urethane ones that are in there as I really didn’t want to re-do that job if I got some diff whine. Anyone want some almost-new rear lower control arms, some new ground control lower control arms, or new bimmerworld solid subframe bushings?

The rear feels normal now, like a regular car without any issues, easy to shift smoothly. Even new many of the e46 M3s had enough lash in the diff that they felt like something was wrong, and this one feels better than my long time ago almost new e46 M3 did. I could also swear that it’s quieter under normal driving and I’m 100% certain that it’s quieter under limited-slip action. So far so good.

I also pinched the steering rack boots to feel for indication of leaks past the seals. No leaks yet, so the rack rebuild continues on strong!
The car really feels great, I’m looking forward to the chassis work to stiffen it up and bring in some of that e39 rigidity feeling. Once the Slon brace, vincebar and v-brace are in, this’ll be an objectively better car than any of the ones out of the factory way back when.

[Seano666]

Good lord, I need that carbon brace in my life!